Ooh this is a tough one! I think I can narrow it down to four:
1. KUHLI LOACHES: I have 18 in my community tank so they all feel safe enough to come out and play all day! They are the funniest little things, always burrowing and digging. They are very social, and even like to interact with my fingers in the water! They also "play dead" in the plants, so cute!
2. MULTIES (SHELL DWELLERS): These tiny cichlids are so cool! They rearrange their home all the time (and get so peeved when I undo their work, so I do it as often as possible hehe!) and dig in the sand all day. They are a colony fish, so they just keep growing their family and take good care of their fry. And they have gorgeous blue eyes!
3. RUMMYNOSE TETRAS: Best schoolers ever! If you have a large enough shoal, you will really see them at their best, with very tight schooling. Plus, they have beautiful coloring! They are the reason I got the size tank I have.
4. RED CHERRY SHRIMP: Not a fish, obviously, but still so entertaining! I have about 70 in my community tank so they are not shy at all. They are always zooming around and getting into all sorts of tight spots. I am actually seriously considering an RCS only tank because I like them so much!